Magnetic storage devices (HDDs) which is capable of reading and writing information have been commonly used in computers. Recently, advantages of such hard disk drives, including large storage capacity, a relatively low prices, fast data access, and high reliability of data retention, have broadened their application to various areas, such as household VCRs, audio appliances, or vehicle-mounted navigation systems. As the application of hard disk drives diversified, a demand for increased density became stronger, and recently, efforts to develop higher-density hard disk drives have been intensified.
At present, magnetic recording media generally mounted in a commercially available magnetic read/write apparatus are in-plane magnetic recording media. In this technique, the easy magnetization axis in the magnetic film is oriented parallel to the substrate. As used herein, the term “easy magnetization axis” denotes the axis along which magnetization is easily directed, and in the case of a Co alloy, denotes the c axis of Co having an hcp structure. In an in-plane magnetic recording medium, when the recording density is increased, the volume per bit of the magnetic film becomes too small, and thus there is the possibility that the read/write performances will deteriorate due to thermal fluctuation effects. In addition, when the recording density is increased, there is a tendency for the medium noise to increase due to the influence of the demagnetizing field at the boundary area between recording bits.
In contrast, what are termed “a perpendicular magnetic recording medium,” in which the easy magnetization axis in the magnetic film is oriented substantially perpendicular to the substrate, is magnetostatically stable even when the recording density has been increased because the influence of the demagnetizing field at the boundary area between recording bits is small. Accordingly, this perpendicular magnetic recording medium has become the focus of attention in recent years as a substitute technology of the in-plane magnetic recording technique. In general, a perpendicular magnetic recording medium includes a substrate, an orientation control underlayer for orienting a magnetic recording layer, a magnetic recording layer made of a hard magnetic material, and a protective layer which protects the surface of the magnetic recording layer. Furthermore, between the substrate and the underlayer, a soft magnetism backing layer which may be responsible for gathering magnetic flux generated from the recording head may be provided.
Even in a perpendicular magnetic recording medium, noise reduction is required while maintaining thermostability in order to achieve higher recording density. A common method of noise reduction is reducing the size of magnetic crystal particles. For example, in the CoCr-based magnetic layer which is widely used at present, the size of magnetic particles are reduced by adding Ta or B, or by making non-magnetic Cr segregated at grain boundaries by heating the particles to a suitable temperature. However, since the size of the magnetic particles has yet to be satisfactorily achieved by segregation of Cr and magnetic crystal particles not being sufficiently separated spatially, magnetic interaction between particles cannot be satisfactory reduced. This, in turn, causes another problem of being unable to sufficiently reduce transition noise between recording bits.
One technique for reducing this magnetic interaction is addition of SiOx to a recording layer so as to form a magnetic recording layer having a granular structure in which magnetic crystal particles are surrounded by the additive (see Japanese Laid-Open Patent Application No. 2002-83411, for example).
Another technique to add an oxide of at least one of alkaline earth metal to a magnetic thin film is disclosed in Japanese Laid-Open Patent Application No. H09-204651.
However, since the diffusion rate of SiOx in a film is low, a sufficient amount of SiOx cannot be segregated at the boundaries of the magnetic crystal particles. As a result, a part of SiOx which has not been precipitated may form a super-saturated solid solution with the magnetic crystal particles, which lowers the crystallinity and orientation of the magnetic crystal particles, resulting in a decreased signal-noise ratio (SNR) of the reading/writing (R/W) performance.